1. Field of the Invention
The present invention relates to a method for producing a rubber-modified styrenic resin, and more particularly to a continuous method for producing a rubber-modified styrenic resin in which molded articles made from the styrenic resin are highly scratch-resistant and impact-resistant, and have a high gloss.
2. Description of the Related Art
Typically, to obtain both high gloss and high impact strength in a styrenic resin, either a rubber polymer is blended in a styrenic resin or, in a state where a rubber polymer is present, styrene is polymerized, and a portion of the styrene in the rubber polymer is grafted while a remaining portion of the styrene, which is a polystyrene-polymerized rubber-modified styrenic resin, is used as material for parts in electrical and electronic products, sheets, etc.
With regard to the rubber polymer used above, it is well known that particles therein are dispersed in the styrenic polymer, with a close correlation existing between a size of these particles and properties such as gloss, rigidity and impact strength of the resulting styrenic resin. That is, rigidity and gloss improve, while resistance to impact is reduced with decreases in the size of the particles of the rubber polymer. An increase in particle size has the opposite effect on these properties. However, there is a limit to improvements in impact-resistance realized with increases in particle size of the rubber polymer.
Accordingly, to improve gloss while maintaining impact strength of molded products, JP 46-41467, JP 59-1519, JP 63-241053 and U.S. Pat. No. 4,146,589, for example, disclose methods of blending impact-resistant styrenic resin having a rubber particle size of 1.0 .mu.m and larger. However, gloss is minimally improved in the rubber-modified styrenic resin produced using these methods.
To overcome the above disadvantage, JP 48-185945 and JP 64-74209 disclose methods for producing a styrenic resin which provides high gloss and high transparency to resulting molded products by using a well-known process [Angew Markromol. Chem. 58/59, p. 157-158(1977)]. In this process, rubber particles of less than 0.5 .mu.m and having a core-shell structure are produced by performing polymerization utilizing a styrene-butadiene block copolymer with a high styrene content as a rubber component. However, surface gloss of molded products manufactured using this material is not greatly improved.
Also, U.S. Pat. No. 4,493,922 and JP 63-112646 disclose methods in which the above styrenic resin is blended with common impact-resistant styrenic resin having a small amount rubber particles with a cell structure to maintain high gloss of the resulting molded product while improving impact strength of the same. In the material resulting from these methods, although impact resistance is significantly improved, since gloss is actually diminished, it is necessary to add polymethylsiloxane to the resin and to either blend the resin in an extruder or mix the resin in a polymerizing reactor with a small and large particle rubber polymerizing solutions. These additional processes complicate the methods.
Further, JP 2-38435 discloses a method for producing a resin using a bulk-suspension polymerization, two-stage polymerization method; EP 412801 discloses a method in which two rubber polymerizing solutions, one with a small particle size and the other with a large particle size, are separately produced in different reactors, then mixed in a subsequent process; and JP 3-199212 discloses a method in which a large particle polymerization solution is produced in a first reactor, and a small particle raw material solution is mixed with the large particle polymerization solution in a second reactor. However, in the above three methods, difficulties in establishing operation conditions for the processes used in the methods arise, and overall costs for manufacture are high.